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2.
CHARACTERISTICS
2.1 WLAN
Wireless LANs (WLAN) were originally designed to be used as an extension or as an
alternative to fixed LANs in order to avoid cabling costs, for example. Today, WLANs are
rapidly gaining significance, as mobile devices, such as laptop computers, are replacing
desktop personal computers. At the same time, major advances in the WLAN technology
have taken place. The overall trend is towards higher bit rates, interoperable networking with
other systems, compatible standards, and interoperable products between different WLAN
product manufacturers.
2.1.1 Frequencies
Traditional cellular systems operate on licensed frequency bands. Operators are given a
certain piece of spectrum from the total spectrum allocation to a particular system for their
sole use. This scheme works nicely with systems covering large areas, such as GSM.
However, given the limited amount of spectrum this means that only a few operators can
have licenses per a given geographical area. For WLANs licensed frequencies clearly are
not a feasible solution since by their nature these systems are operated, for example, by
companies or individuals in a limited geographical area, such as a building. Therefore, pico
cell networks and personal area networks operate on unlicensed spectrum. This means that
a certain piece of spectrum is available for a set of systems/technologies, provided that they
fulfil certain agreed, very basic requirements, like maximum allowed transmit power. This
means that the operating environment, and the related problems, are very different
compared to traditional cellular networks.
2.1.2 Standards
IEEE initiated the 802.11 /24/ working group in 1990 to develop a WLAN standard for the 2.4
GHz unlicensed ISM (Industrial, Scientific, and Medical) frequency band that is available
globally (with some national and regional limitations). The IEEE 802.11 standard was
approved in 1997, and the first products appeared roughly at the same time.
Like IEEE 802.3 (Ethernet) or 802.5 (Token Ring) also 802.11 only covers the
two lowest layers of the OSI reference model. 802.11 reference model consists
of two main parts, MAC (Medium Access Control) and PHY (Physical) layers.
MAC functions on the Data Link layer and PHY on the Physical layer of the OSI
model.
Currently, the IEEE 802.11 family of standards includes specifications for five different types
of physical layers. Four of these are based on radio technologies and one on diffuse infrared
technology. All physical layer alternatives utilise the same MAC protocol. Three of the radio
PHY standards utilise the 2.4 GHz frequency band and one is designed for the 5 GHz
frequency band.
IEEE 802.11b /25/, the current industry standard that the vast majority of the current WLAN
products -- including the Nokia D211 – support, specifies a 2.4 GHz radio that utilises Direct
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